The present invention relates to a fluid flow measurement device, particularly to a momentum flux device for use in the metering of fluid flow in venturi tubes.
Fluid flow measurement devices based on the principle of momentum flux measurement are known in the art as pressure drop measurement devices. Examples of these are orifice plate, nozzle and venturi tubes described in ISO 5167. A venturi tube works on the principle that the flow of fluid through a body exerts a pressure on the body, the pressure being dependent on the rate of fluid flow. The venturi tube comprises a pipe formed with a constriction or xe2x80x9cthroatxe2x80x9d. The flow velocity of fluid in the body increases as it flows through the throat. The portion of the pipe downstream of the throat is tapered so that the pipe cross section returns gradually to its value upstream of the throat. The pressure exerted by the fluid (measurable by a pressure measurement means such as a U-tube manometer) in the region of the throat is correspondingly less than that in the region upstream (or downstream) of the throat. The difference in pressure between the two regions of the venturi tube depends on the flow velocity and thus depends on the quantity of fluid passing through the pipe per unit time. Hence this pressure difference provides a measure of the fluid flow rate.
One problem with conventional fluid flow measurement devices based on the principle of momentum flux measurement, or pressure drop measurement, is that they have a somewhat limited operating range. Thus, it can be difficult or impossible to measure accurately some ranges of fluid flow rates. One reason for the limited range of conventional devices is that a predictable and stable correlation between measured pressure drop across the device and the momentum flux of a fluid flowing within the device requires the fluid to have certain flow parameters within a certain range. In particular, the Reynolds number, which expresses the ratio between momentum and the viscous forces in the fluid flow, must be within a certain range. For a given fluid or mixture of fluids this requires the fluid velocity to be within a certain range. Another limitation on conventional pressure drop measurement devices is that they rely on the quantification of a pressure differential caused by fluid flowing at different velocities in different parts of the device. For certain flow rates, the pressure differential created by a conventional device may be too low, or too high, to be detectable or accurately measurable by the differential pressure sensor. Operating range may also be limited by the maximum pressure loss acceptable for the process in which the device is installed, as the device pressure loss is associated with the pressure drop across it.
Therefore, measurement using conventional devices of fluid flow rates which are subject to fluctuation outside the operating range of the device installed requires the replacement of the measurement device by another suitably sized device capable of measuring the range of fluid flow rates in question. For multi-phase flow of unprocessed well streams in particular, the range of flow conditions and the operating range required are normally very large and often require more than one conventional measurement device in order to cover the whole range of fluid flow characteristics.
The removal and replacement of a metering device requires the fluid flow to be stopped or rerouted during the operation. This may for a variety of reasons be impractical and costly, particularly if the device is located in a remote off-shore or sub-sea area. Furthermore, if the device forms part of an integrated metering system, such as a multiphase flow metering system, removal and replacement of the device can affect the calibration of the system.
The present invention provides a device for measuring fluid flow rates having a body defining a conduit for the fluid, the body having selectable first and second configurations, wherein in the first configuration the body presents a first constriction in the conduit for measuring a first range of fluid flow rates, and in a second configuration the body presents a second constriction in the conduit for measuring a second range of fluid flow rates, the second constriction being narrower than the first constriction, wherein the body is arranged for operative connection to means for determining the pressure difference between each constriction.
The device of the invention operates, in its first configuration, in the same way as a conventional venturi tube. When the flow rate of the fluid decreases below the range at which the pressure difference between the conduit and the first constriction can be measured accurately, the device of the invention may be switched to its second configuration. This switching may be done manually or by remote control. In its second configuration, a second pressure difference is created between fluid flowing in the first constriction and fluid flowing in the second constriction. This second pressure difference provides a means for measuring fluid flow rates outside the operating range of the device in its first configuration.
It is envisaged that for most practical purposes the provision of two pressure differences as described in the second configuration of the device of the invention will be sufficient to measure fluid flow rates over a sufficiently wide range of flow velocities and fluid density characteristics. However, it is also possible to provide more than two, for example three, four or five pressure differentials in third, fourth and fifth configurations of the device.
It is preferred that the or each constriction has a portion of constant transverse cross sectional area to facilitate pressure measurement in the region of the respective constriction. The pressure measurement means may comprise independent manometers for measuring the fluid pressure in the region of the first and second constrictions and in the main conduit. It is also preferred that the conduit has a tapered portion between the first and second constrictions in the second configuration of the device.
In one preferred embodiment of the invention, the device includes an annular member arranged to be moveable within the body to form the second constriction. The annular member may be moveable longitudinally of the conduit. In this case, the annular member preferably has an outer surface shaped to be received within the conduit of the device in snug fit arrangement with the body. Still more preferably the annular member and the body have cooperating portions for providing a snap-fit arrangement in the second configuration of the device.
Alternatively, the annular member may be moveable radially of the conduit. In this case, the annular member may comprise two or more sections which are separable, the sections being separated in the first configuration of the device and being united in the second configuration of the device. Preferably, the annular member has three sections. The annular member may, in the first configuration of the device, be located in a housing surrounding the conduit.
In a preferred embodiment, a body of the device comprises an openable portion which is moveable between a closed position in the first configuration of the device and an open position for allowing insertion of the annular member to form the second configuration of the device. One or both of the annular member and the openable portion may be moveable by hydraulic means and/or electronic means. It is preferred that first biasing means be provided for biasing the annular member radially inwardly towards a position at which the second configuration of the device is formed. Second biasing means may be provided for biasing the openable portion towards its closed position. The or each biasing means may be a compression spring.
Conveniently, in the first configuration of the device, the annular member bears upon the openable portion in its closed position under the influence of the first biasing means. First urging means may be provided for urging the annular member radially outwardly so that it no longer bears upon the openable portion. Second urging means may be provided for urging the openable portion towards its open position. The or each urging means may be hydraulically operable.
In one preferred embodiment, the openable portion is a telescopic portion of the body of the device.
It is also preferred that the annular member includes a pressure tapping for communication between fluid flowing in the conduit of the device in its second configuration and the third pressure measurement means.
Also preferred is that the annular member be provided with one or more sealing members for sealing the annular member in fluid-tight engagement with the body in the second configuration of the device.
Accordingly, the invention relates to a remote control or manually insertable mechanical constriction for a venturi tube in order to create an additional pressure differential in the tube. This is achieved because the momentum flux in the fluid flow is increased in the second constriction of the conduit in relation to the first constriction. A higher differential pressure is thereby provided allowing accurate and stable measurements at low flow rates or at certain density conditions to be achieved. The device of the invention increases the operating range of the conventional venturi tube with a minimum of human interference.